A device is provided for simultaneous determination of forces, comprising an arrangement of parallel springs with integrated piezoresistive resistances in the form of a full wheatstone bridge. The parallel springs, have, for example, silicon or fused quartz, are arranged in lines, wherein the parallel springs at their ends are permanently connected by silicon or fused quartz spacers. At the free ends of the parallel springs, measurement-technology elements are arranged. The opposite ends of the parallel springs are fastened to a frame.
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1. A device for measuring forces with a parallel spring arrangement, the device comprising:
a plurality of parallel springs arranged in a line;
spacers, the parallel springs being connected together at their ends over the spacers, wherein the parallel springs and the spacers are made of silicon;
a frame for fixedly holding an end of each of the parallel springs;
measurement-technology elements arranged on free ends of the parallel springs; and
a set of four piezoresistive resistances located in each of the parallel springs, each set being arranged together in a corresponding full wheatstone bridge.
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This nonprovisional application claims priority under 35 U.S.C. §119(a) to German Patent Application No. DE 10 2007 033 441, which was filed in Germany on Jul. 18, 2007, and which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a device for measurement of forces with a parallel spring arrangement. The invention is particularly suitable for the determination of forces and masses of small objects. The device in accordance with the invention can also be used for simultaneous measurement of surface profiles at several points.
2. Description of the Background Art
Various arrangements are known from the conventional art for force measurement or mass determination of objects and for measurement of surface profile deviations.
In numerous applications, bendable elements with strain gages applied to them are used for force measurement. For higher requirements in force measurement and weighing technology, deformation element parallel springs, for example made of aluminum, are used. Strain gages are likewise glued onto the surface of these deformation element parallel springs. Since the deformation element parallel springs exhibit both expansion and compression zones, the strain gages can be evaluated in full-bridge circuits. One drawback in this case is that the manufacturing of the strain gage force sensors requires a technologically intricate process. An additional drawback is the limited accuracy of these force sensors, especially for the measurement of small forces.
For determining the surface profiles of objects, silicon cantilevers in atomic force microscopes with resolutions in the nanometer and subnanometer range are used. The cantilevers used are silicon bending beams with piezoresistive resistances, which are connected into a bridge. Since these bending beam cantilevers have only positive or negative length expansions, full-bridge circuits cannot be realized. An ideal full-bridge circuit with piezoresistive resistances is only possible with a parallel spring arrangement.
It is also known to use cantilevers which have reflective surfaces at their free ends, the deflection of which is detected with flat-mirror interferometers. However, because of the inclination of the bending beam cantilevers at their free ends, flat-mirror interferometers can be used only at very small elongations and compressions.
It is therefore an object of the present invention to provide a device that makes possible a substantially simultaneous ascertainment of forces and a precise determination of the profile deviations of surfaces at several points simultaneously.
In an embodiment, the device comprises a parallel spring arrangement, especially made of silicon or fused quartz. The parallel springs include upper and lower springs that are connected by spacers made of silicon or fused quartz. One end of each of the parallel springs is fastened to a frame. Because the parallel springs have an inflection point in the course of their bending, there are zones in which the absolute values of the expansions and compressions are equal. In this way it is possible to realize an ideal full Wheatstone bridge with piezoresistive resistances in all parallel springs.
Parallel springs having silicon can be manufactured cost-advantageously and with high reproducibility by means of semiconductor technology and micromechanics. In this case, linear arrangements of silicon parallel springs can advantageously be manufactured even in large quantities. If the silicon parallel springs do not have a rectangular shape, but rather a triangular shape, the mechanical elongations and stresses in the triangular areas are practically constant. The silicon parallel springs are advantageously made in double triangular form, i.e., the parallel springs have a kind of butterfly shape. This results in position independence of the piezoresistive resistances and thus an offset-free bridge circuit. If the silicon parallel springs have a rectangular shape, the mechanical elongations and stresses have gradients. Therefore, the piezoresistive resistances should be positioned exactly to keep the offset stress of the bridge low. This is generally not adequately possible.
If silicon spikes with radii of only a few nanometers are attached to the free ends of the silicon parallel springs, parallel spring cantilevers are formed which in contrast to the bending beam cantilevers of today's atomic force microscopes have no so-called arc errors. This is an additional essential advantage of the arrangement in accordance with the invention.
In this way, object surfaces at several different points on the object can be scanned and the surface profile deviations ascertained with the highest possible precision.
If silicon probe elements are located on the free ends of the silicon parallel springs both on the top side and at the opposite points on the underside, for example, diameters of holes and the distances between grooves or bars can be measured.
If weighing pans are attached to the free ends of the parallel springs and the objects to be weighed are passed over the pans in a suitable way using delivery and removal devices, the masses of the objects to be weighed can be determined in parallel with high effectiveness.
The parallel spring arrangements made of fused quartz can be manufactured economically with the aid of etching technology and modern precision machinery. At the free ends of the fused quartz parallel springs, spikes or probe elements made of hard metal or diamond and with different radii can be attached. The deflections of the fused quartz parallel springs are advantageously measured optically. For this purpose, autofocusing sensors or flat mirror interferometers can be used. The combination of fused quartz parallel springs or silicon parallel springs with a flat mirror interferometer guarantees the highest possible precision.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:
Corresponding parts are provided with the same reference numbers in all figures.
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The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.
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